Abstract
An in-vacuum undulator (IVU) provides a means to reach high-brilliance x rays in medium energy storage rings. The development of short period undulators with low phase errors creates the opportunity for an unprecedented brilliant light source in a storage ring. Since the spectral quality from cryogenic permanent magnet undulators (CPMUs) has surpassed that of IVUs, NdFeB or PrFeB CPMUs have been proposed for many new advanced storage rings to reach high brilliance x-ray photon beams. In a low emittance ring, not only the performance of the undulator but also the choice of the lattice functions are important design considerations. Optimum betatron functions and a zero-dispersion function shall be provided in the straight sections for IVU/CPMUs. In this paper, relevant factors and design issues for IVUs and CPMUs are discussed together with many technological challenges in short period undulators associated with beam induced--heat load, phase errors, and the deformation of support girders.
Highlights
Undulators are the main insertion devices to generate high brightness radiation in synchrotron radiation (SR) facilities
A cryogenic permanent magnet undulators (CPMUs) becomes a favorite for short period undulator developments because of its attractive characteristics: (1) 20%–30% higher field compared to an in-vacuum undulator (IVU) with the same magnet circuit design, (2) high resistance against radiation damage, (3) easy extension from IVU design, (4) reduction of image current heating on a magnet cover by factor of 2–3 compared to an IVU and (5) a manageable heat budget up to several hundred watts, which is much higher than that in a SCU
Development of short period IVUs/CPMUs is a recent trend for x-ray sources
Summary
Undulators are the main insertion devices to generate high brightness radiation in synchrotron radiation (SR) facilities. The imperfection of the magnetic field causes such a fluctuation of the phase difference degrading the desired constructive interference This specification is important for 3-GeV facilities where the utilization of higher harmonics of undulator radiation is essential to reach hard x rays. Considering the resistive wall instability, additional caution is necessary because the typical bunch length in XFEL linacs is typically a hundred times shorter compared to the 10 psec in storage rings Such short linac bunches may create energy variations in the electron beam through interaction with the narrow undulator vacuum pipe [16]. Low phase error issues and optimal lattice functions for an undulator are discussed to maximize the undulator performance for low emittance storage rings
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